Typical applications for slurries are the scrubbing of facility exhausts. For example, the exhaust of a coal-fired facility would typically contain acidic flue gasses and fly ash. Both pose environmental problems.
The acidic flue gasses are predominantly sulfur dioxide (SO2) and hydrogen chloride (HCl). These acidic flue gasses combine with moisture in the atmosphere to produce sulfuric and hydrochloric acids, which then precipitate as acid rain.
Fly ash is the finely divided residue that results from the combustion of ground or powdered coal. Fly ash is a pozzolan, i.e., a siliceous material which, in the presence of water, will chemically combine with calcium oxide (lime) to produce a cementitious material. If allowed to pass into the atmosphere, fly ash would precipitate, either directly or in combination with atmospheric moisture, and form clogging films, scums, and sediments.
To inhibit the environmental consequences of such an exhaust, it is desirable to remove acidic flue gasses and fly ash from the exhaust before releasing the exhaust into the atmosphere. This is done in a scrubbing process, typically with the use of a lime-based slurry.
Sulfur dioxide reacts with lime to form calcium sulfite, which can then be air-oxidized to form calcium sulfate dihydrate (gypsum):SO2+Ca(OH)2→2CaSO3+H2O2CaSO3+O2→2CaSO4CaSO4+2H2O→CASO4·2H2O
Similarly, hydrogen chloride reacts with lime to form calcium chloride:2HCl+Ca(OH)2→CaCl2+2H2O
Fly ash combines with the lime in the slurry to form siliceous compounds having cementitious properties. Being cementitious, these compounds tend to agglutinate into masses within the slurry.
While removing the acidic gasses and fly ash from the exhaust, the scrubbing process contaminates the slurry. The byproducts of the scrubbing process, calcium sulfite, calcium sulfate dihydrate, calcium chloride, and various siliceous compounds, enter and become a part of the slurry. These byproducts may be removed from the slurry by various chemical and/or separation techniques known to those skilled in the art.
The slurry is typically recycled and refreshed for both environmental and fiscal considerations. This requires that characteristics of the slurry be adjusted appropriately.
There is, therefore, a need to control the characteristics of a lime-based slurry used in the scrubbing of the exhaust of a coal-fired facility. This is often accomplished by monitoring a desired characteristic either periodically or substantially continuously, and adjusting the slurry in response to this monitoring. For example, if the desired characteristic is slurry density and if monitoring indicates the slurry is too dense, then more water may be added. Similarly, if the monitoring indicates the slurry is insufficiently dense, then more lime may be added. Such adjustments, in conjunction with contaminant removal, allow a slurry to be used indefinitely, with minimal wastage of both the water and the lime.
In scrubbing the exhaust of a coal-fired facility, the acidic flue gasses and fly ash pass into the slurry. This results in a slurry that is acidic, abrasive, and lumpy, with some “lumps” (agglutinations of siliceous compounds from fly ash) attaining a significant size and mass.
The contaminated slurry attacks the structure used to contain it, especially when the slurry is in motion. The walls and piping containing the slurry are continuously eroded, abraded, and pounded by the slurry. This results in further contamination of the slurry by components of the containment structure. A typical slurry therefore may not only contain water and lime; but also sulfur dioxide, hydrogen chloride (hydrochloric acid), and fly ash directly from the exhaust; sulfuric acid, calcium sulfite, calcium sulfate dihydrate (gypsum), calcium chloride, and agglutinations of cementitious siliceous compounds as exhaust byproducts; and even metal fragments, fiberglass, pieces of concrete, rebar, and chevrons from the containment structure itself. This provides an extremely hostile environment into which to place a monitoring device (e.g., a density meter). Filters, filter housings, small-bore piping, and monitoring devices used in such an environment are often clogged, damaged, or destroyed by the contaminated slurry.